As is well known, lignin is a complex, high-molecular weight polymer occurring naturally in close association with cellulose in plants and trees. Lignin constitutes, on a dry weight basis, approximately 27% to 33% of the tree in softwoods and approximately 20% to 24% in hardwoods. In the paper-making industry, lignin is recovered as a by-product of the cellulose product by two principal pulping processes known as the sulfite process and the kraft process. In the sulfite pulping process, lignin is solubilized from the cellulosic portion of the wood pulp by direct sulfonation, while the kraft process is based on an alkaline degradation mechanism causing cleavage of .beta.-aryl ether linkages in the polymeric lignin which subsequently results in chemical functions of the phenolic and carboxylic type. Kraft lignin is isolated by acid precipitation from the black liquor of a kraft pulping process at a pH below the pKa of the phenolic groups.
The high degree of chemical activity which is characteristic of lignin salts permits the preparation of many novel and economical organic derivatives. Typical reactions which lignins can undergo are hydrogenation, halogenation, nitration, sulfonation, oxygenation, salt formation, etherification, and esterification. Lignin by-products variously have been employed in various chemical compositions as a surfactant, extender, dispersant, reinforcement, absorbent, binder, sequestering agent, emulsifier and emulsion stabilizer, and as a stabilizing and protective colloid. Lignosulfonate compounds, particularly sodium salts of lignosulfonates, have been employed as additives and dispersants in textile dyestuffs and printing pigments. Sodium salt sulfonated lignin by-products have been sold for many years under the trademark Indulin.RTM. by Westvaco Corporation of North Charleston, S.C.
Reduction in the pH of black liquor containing soluble lignin salts generally has been accomplished by introduction of carbon dioxide which converts the phenolic hydroxyl groups on the lignin molecule, which are in ionized form, into their free phenolic or acidic form. This conversion renders the lignin insoluble in the black liquor, and, as a result, it precipitates out. To precipitate the alkali lignin from the black liquor, the pH of the black liquor, initially around 13, is lowered to a pH of about 10.5 at which point lignin precipitation begins. Lignin obtained from the kraft process is not recovered as a sulfonated by-product, but is sulfonated, if desired, by reacting the material with a sulfur and oxygen-containing compound. Sulfonated lignins are understood to be those containing at least an effective amount of sulfonate groups to give water solubility in moderately acid and higher pH solutions.
One conventional process for sulfonating kraft lignins involves sulfomethylation of the alkali lignin by reacting the lignin with sodium sulfite and formaldehyde. Such a process is described in Adler, et al. U.S. Pat. No. 2,680,113. Recently, it has been proposed to sulfomethylate kraft process lignins to produce a low electrolyte-containing product in a two-step lignin isolation where the ionized phenol component of the lignin is methylolated after precipitation isolation from the black liquor, in alkaline solution by the addition of an aldehyde, the pH is then lowered to acid to precipitate the methylolated lignin and wash the precipitate to remove undesired inorganic salts and electrolytes therefrom, and the lignin thereafter sulfonated by addition of a salt, typically sodium, and also ammonium, of a sulfur and oxygen-containing compound. Such processes are described in commonly assigned U.S. Pat. No. 4,590,262 which issued May 20, 1986, and U.S. Pat. No. 4,642,336 which issued Feb. 10, 1987.
More recently, it has been proposed to produce sulfomethylated lignin amine salts of the lignin by-products of the black liquor residue of a kraft wood-pulping process. Such amine salts of lignosulfonates are particularly suited for use as additives in dyestuffs, pesticides, and other chemical compounds, and their production is disclosed in co-pending commonly assigned U.S. patent application Ser. No. 06/783,781 filed Oct. 3, 1985. It has been found that sulfonated lignin salts containing a cation having a relatively low disassociation constant and pKa, such as the amine salts, have less tendency when used as a dispersant to cause azo dyestuff reduction and less color loss than is the case in the use of the higher pKa lignin salts, such as sodium or ammonium. Such products are also formed by a two-step lignin isolation/filtration process whereby undesired inorganic salts and electrolytes are removed by water wash of the methylolated lignin before sulfonation to produce low electrolyte-containing sulfomethylated lignin salts causing less azo dye reduction and color loss when used as dispersants in azo dye systems.
Generally, in preparation of methylolated lignins from the black liquor of a kraft pulping processes as described above, it has been the practice to isolate the lignin recovered from the black liquor by an acid precipitation and filtration step, and to thereafter react the filtered recovered lignin by-product in alkaline solutin at a pH level of about 9 to 11 with an aldehyde compound to carry out methylolation.
More recently, as set forth in co-pending commonly assigned U.S. patent application Ser. No. 819,600 filed Jan. 17, 1986, an improved method for preparing low electrolyte sodium lignosulfonates has been proposed wherein the isolated lignin material precipitated from the black liquor of a kraft pulping process is methylolated in alkaline liquid medium, and the pH of the liquid medium lowered to the acid range by addition of sulfur dioxide to precipitate the same, and thereafter the precipitated methylolated lignin is heated to sulfonate the same. Such improved method of producing low electrolyte-containing sulfomethylated lignin products involves only one lignin isolation step in its recovery and processing from the black liquor, instead of two, as had been practiced heretofore.
Commonly assigned U.S. Pat. No. 4,629,469 discloses a method of producing a product of the black liquor residue of kraft pulping process which may be used in conjunction with lignosulfonate salt dispersants to provide humectant properties to dyestuff compositions. The process described in the patent involves the steps of oxidizing the black liquor residue of a kraft pulping process, methylolating the lignin component of the oxidized liquor residue, adding sulfur dioxide to the methylolated lignin-containing oxidized residue to adjust the pH to the range of between about 2 to 10, adding equal molar quantities of formaldehyde to the amount of any residual sodium sulfite in the residue, heating the resultant residue under sufficient conditions to sulfomethylate the lignin components of the residue, and concentrating the residue to a solids content of between about 50% to 70% with adjustment of the pH to about 7.5. The black liquor residue product may then be effectively used as a humectant additive, along with a dispersant, such as a sulfomethylated lignin salt produced by the lignin isolation methods described above, in dyestuff compositions.
It can be appreciated that in production of methylolated lignins and sulfomethylated lignins for use as primary dispersants in dyestuff compositions, it is most desirable to minimize the number of lignin isolation steps involved in the recovery and manufacturing process. In the processes hereinabove described for producing sulfomethylated lignin salts as dyestuff dispersants, it has been the practice to first isolate the lignin material from the black liquor of a kraft pulping process before its methylolation, necessitating the use of pH-lowering chemicals to reduce the pH of the black liquor to the lignin precipitation point for recovery of the same, followed by the addition of pH-raising chemicals for carrying out the methylotion step. Such reduction of the acidity of the black liquor before methylolation obviously involves an added cost in the recovery and preparation of the lignin by-product suitable for use as chemical additives in other chemical processing operations, such as dispersants and additives in dyestuffs and pigments. If methylolated lignins recovered from kraft black liquor are to be used as intermediates in other chemical manufacturing processes, it is likewise desirable that the recovery process steps of lignin isolation be minimized for economics of manufacture.